In the United States, FM broadcasters can transmit information in sidebands within 100 kHz of their assigned carrier frequency at full power, and from 100 kHz to 200 kHz around the carrier at 30 dB down from full power. The standard stereo audio signal is placed in a bandwidth within 53 kHz of the carrier. The broadcaster is, thus, able to transmit other information in the remainder of the bandwidth, subject to the constraints described above.
It has become desirable for FM broadcasters to simultaneously broadcast stereo audio and digital data. The digital data could, for example, represent a high quality version of the stereo audio being broadcast. This requires a relatively high data rate channel which is restricted to a relatively narrow bandwidth. For example, a digital data stream carrying high quality audio can have a bit rate of 128 kilobits per second (kbps). A signal carrying such a data stream cannot be transmitted in the bandwidth available in an FM broadcast signal without some form of compression to decrease the bandwidth required for the signal.
It is always desirable to provide data at higher data rates through channels which have limited bandwidth. Many modulation techniques have been developed for increasing the data rate through a channel. For example, M-ary phase shift keyed (PSK) and Quadrature Amplitude Modulation (QAM) techniques permit compression by encoding a plurality of data bits in each transmitted symbol. Such systems have constraints associated with them. First, the hardware associated with such systems is expensive. This is because these techniques require a high level of channel linearity in order to operate properly. Consequently, extensive signal processing must be performed for carrier tracking, symbol recovery, interpolation and signal shaping. Second, such techniques are sensitive to multipath effects. These effects need to be compensated for in the receiver. Third, these systems often require bandwidths beyond those available in some applications (for example in-band on-channel broadcast FM subcarrier service) for the desired data rates.